1. Field of the Invention
The present invention relates, in general, to magnetic tape media used for data storage and manufacture of magnetic tape, and, more particularly, to tape manufacturing methods and systems, such as servo writers, that are useful in producing high quality magnetic tape including written servo patterns using a supply or feed of magnetically unoriented tape media.
2. Relevant Background
Magnetic tape has been used for data storage for many years. Modern usage typically involves magnetic tape that is packaged in cartridges or cassettes being used for backups and archives. In part, this use has evolved because when storing large amounts of data, tape can be substantially less expensive than disk or other data storage options. The device that performs the actual writing and reading of data is a tape drive, and autoloaders and tape libraries are often used to automate cartridge handling. Magnetic tape allows massive amounts of data to be stored for long periods of time while providing relatively rapid access to archived data.
Magnetic tape is a medium for magnetic recording that is made up of a thin magnetizable coating on a long and narrow strip of plastic (e.g., PET, PEN, or the like) or other substrate material. For example, the magnetic media coating may be composed of up to 50 percent or more of magnetic particles by volume, a polymer binder, and small amounts of cross-linking agents, dispersants, and lubricants. With advancements in tape formulations, coatings are becoming thinner and particles are becoming smaller to allow for higher linear density (e.g., bits per inch), which drives the need for new formulations and coating techniques. In the past, half-inch and other width magnetic tapes were fabricated with iron oxide being the magnetic particles but a switch was made to use chromium dioxide for these particles in the magnetic coating. More recently, tape formulations have made use of magnetic particle technology called Metal Particulate (MP) with MP providing a much higher coercivity (i.e., the ability to resist internal demagnetization) and smaller particle lengths. MP-based tapes provided more desirable signal-to-noise levels for higher density reading and writing on the magnetic tapes such that data capacities have grown up to 1 TB with some technologies anticipating capacities to grow to 3 TB or more per magnetic tape cartridge.
As part of manufacturing magnetic tape media, a servo pattern is written on the media prior to winding it into a shell or cartridge housing and later use in a tape drive to write and read data. The servo pattern may take numerous forms to suit a particular product type, but, in general, the servo pattern is used by the tape drive to ensure precise alignment between the drive head and the tape's data tracks. The servo patterns (or signals) may be written by a servo writer that uses a head or servo write head by recording a pattern into the tape. During use of the magnetic tape, the servo patterns are read by a magnetic head while the position of the magnetic head relative to the magnetic tape (e.g., the position along the width of the tape) is servo-controlled. To allow the drive and/or a servo signal sensing element to read the servo pattern or signal correctly, it is important for the pattern or sensed signal to provide a signal (e.g., a voltage signal) that has a relatively high amplitude (e.g., to maintain a desirable signal to noise (SN) ratio) and that is substantially symmetric.
Presently, tape media is magnetically oriented media. The media is typically comprised of acicular magnetic particles that are physically aligned in the downtrack direction of the tape during manufacturing. The acicular nature of the particles has a strong shape anisotropy that yields a large magnetic moment in the preferred direction of the shape of the particle (along the longest axis, for example). The intrinsic shape anisotropy coupled with the mechanical alignment of the particles creates an assembly of particles that do not have a significant magnetic component perpendicular to the surface of the tape or a perpendicular component. Prior to servo writing, the oriented media may be “erased” with AC degaussing techniques. Additionally, some media manufacturing processes utilize a DC erase head to apply a magnetic field to one side of the tape to DC erase the tape, e.g., to magnetize the magnet tape in one direction such as along the length of the tape. Specifically, forward magnetization assists in increasing the strength or amplitude when a servo pattern is later written to the oriented magnetic tape.
More recently, the data storage industry has been developing magnetic tape that is not magnetically oriented when manufactured (or is magnetically unoriented tape) in attempts to increase the capacity or areal density of tape-based storage products such as tape cartridges. Oriented tape has higher inherent noise due to the traditional shape anisotropy that could be addressed by use of AC erasing, and, in some cases, single-sided DC erasing to arrange the magnetic particles in a more uniform manner (such as with forward magnetization along the longitudinal axis of the tape) prior to servo writing so as to increase the amplitude of the read servo signals. Such techniques have not been useful with magnetic tape recording media that is randomly oriented because the media relies on crystalline anisotropy, rather than shape anisotropy for good recording characteristics. Randomly oriented media thus has the potential for significant perpendicular components to its magnetic fields (e.g., a magnetic component that out of the plane of tape or the tape thickness). Hence, existing processes involving AC or DC erasure have not been effective in producing media that can readily retain a servo pattern with high amplitude and acceptable symmetry.
Since the properties of the unoriented magnetic tape have caused difficulties in writing timing-based servo patterns that have high amplitude and low distortion, there remains a need for an enhanced method of writing good quality servo patterns on tape media in which the magnetic film or coating is not magnetically oriented as part of manufacturing. Preferably, such methods would be useful with existing servo writing processes, such as by implementation as a pre-servo writing stage or station within a servo writer.